Synthesis and electrochemical studies of layer-structured metastable αI-LiVOPO4

The layer structured alpha(I)-LiVOPO4 was obtained via a two step chemical synthesis. In the first step, a hydrated phase, LiVOPO4 center dot 2H(2)O, was obtained by a simple hydrothermal route at 120 degrees C. Single crystal X-ray diffraction analysis revealed the structure of LiVOPO4 center dot 2H(2)O to be orthorhombic with lattice parameters: a = 8.9454(7) angstrom, b = 9.0406(7) angstrom and c = 12.7373(10) angstrom. Dehydration of the parent compound led to its structural transformation to tetragonal alpha(I)-LiVOPO4, which was only identified previously during the lithium insertion in VOPO4. We have investigated the solid-state dehydration of LiVOPO4 center dot 2H(2)O and proposed a possible mechanism for the crystal structure transformation. Electrochemical characterization of this rarely studied tetragonal phase revealed its good lithium cycling at high operating voltage. Galvanostatic charge-discharge cycling of alpha(I)-LiVOPO4 was studied in a voltage window of 2.5-4.5 V, which shows a stable reversible capacity of 103(+/-3) mA h g(-1) at a current density of 16 mA g(-1) (0.1 C). At higher current rates, although it exhibited good cyclability, the capacity was found to decrease with increasing current rates. The long term cycling stability of the above material was demonstrated at a current rate of 0.5 C up to 200 cycles.